Most Canon EOS shooters buy a 50mm f1.8 prime because it's cheap, fast and sharp. I had a chance to use one on two occassions, but I found that I had difficulty composing with it as I have a crop APS body such that the 50mm length is more like 80mm. This is great for portraits in a studio but terrible for event photography.

The Sigma 28mm is a great alternative: The 28mm length is easier to handle and compose with and the maximum aperture of f1.8 is great for indoors shots without flash. The lens features a large front element diameter which really lets light in and makes the lens quite bright.

Shooting at f1.8 on any lens is tough because the depth of field is so shallow. However, if you need the range in low light, it's useful. Also, the lens can be very sharp with enough light.

The focusing mechanism disengages the clutch when the focus ring is pulled forward, which allows you to keep your fingers on the ring while the lens is in AF. When you toggle the lens to manual, you can pull the ring backwards, which engages the clutch and lets you focus manually. Note this lens uses a traditional servo motor, so it does not have FTM like the Canon USM lenses.

The lens is also really nice for macro photography.

Sigma includes a "perfect" lens hood and a nylon padded carry bag, which are nice touches. The lens has a nice matte finish and the focus ring is easy to grasp.

There are a few downfalls, however. The large 77mm diameter is great, but it also means expensive filters. The Sigma 28mm is also quite heavy in comparison to the Canon EF 28mm.

The motor is louder than most and it is not that quick in focusing. However, for the price, it's just fine. Highly recommended for those who want a fast, wide lens for a reasonable price compared to the Canon EF 28mm.

This is an ideal lens for travelling and general purpose photography with the EF-S APS type cameras, such as the EOS300D, 350D, or 20D.

I bought my EOS300D with the kit 18-55mm. It was not a bad choice as I needed to learn the limitations of the basic lens before stepping up to a better one. The 17-85mm is ideal for amateurs who want something more than the kit lens, but can't justify a L series lens selection's price tags:

-Stabilization for lower light situations
-Longer zoom than 55m for portaits and closeups
-Wideangle for landscapes

The most important feature of this lens is the range: You could buy the 28-135mm IS USM, but on a crop body, 28mm is not wide enough. I got used to shooting at 18mm on the kit lens, so the 17mm wide angle is important to me, especially when travelling for shooting landscapes and getting in scenery. 55mm is fine for most day to day shooting, but 85mm is very useful for portraits and also to pick up details while on the road.

IS is my favourite feature of this lens. I don't like using flash and I can't see myself lugging a tripod around on vacation, so the stablizer is crucial. It really does work as advertised, BUT with one caveat: You can stablize the camera, but you can't stablize the subject. So IS is great for inanimate objects in the distance or in the dark, but not for people dancing or kids jumping around in those situations. Regardless, the Image Stablizer is one of the major reasons why I like this lens.

The USM focusing is icing on the cake, but not critical. The 17-85mm supports FTM.

The major downside is the price of this lens. It's almost six times the cost of the kit lens and not widely found like the 28-135mm. So you are paying a premium for the EF-S mount, which you may upgrade away from in the future. I can't see myself being able to afford a full frame body for years, so I don't think it's an issue for me, but you'll have to make that call yourself.

I'm not stringent on image quality as some, but I did notice CA at the wide st angle of this lens. The lens also appears fragile with the IS components inside.

I also wish that Canon would throw in a lens hood and pouch the way Sigma does with their lenses. Even the cheapest Sigma lens comes with an included lens hood. After buying such an expensive lens (at least for my budget) it's annoying to have to pay more for what is a very cheap part to manufacture.

In summary, a good choice as an upgrade from the basic EF-S package lens.